I don't use a standalone hygrometer, but I do use an Incutherm thermometer/hygrometer combo with probe and find the hygrometer to be pretty accurate (better than my incubators' displays, for sure). I also have aquarium thermometers and recently purchased a Brinsea Spot Check, too.
I think accurate temps and humidity are important, but I'd say it's more important that
temps be accurate - too hot and it can prove lethal, too cold and development will (at the very least) be delayed. I've found most chicken/duck/guinea/goose eggs more forgiving of humidity fluctuations - slight humidity variations usually don't have such pronounced effects as temperature fluctuations (not including certain types of eggs that require specific humidity conditions).
If you search BYC, there are multiple threads on the topic, like
this recommendation from
@Pyxis. Good luck!